Add GStatus module for "is" methods

[coloursofnoise]

Once #535 or similar is merged, I would like to propose adding a "GStatus" (or similar) module generated by the clang scripts to encompass "is" functions such as gtk_widget_is_focus that are not currently covered by the GAccessor module.
This could be fairly easily tacked onto the current gtk_get_set_gen.jl script and generated into gbox.

Feedback and other input is appreciated.

[tknopp]

sure, the more is covered, the better. You just have to look, if we wrapped one _is_ function manually and replace that with the one being wrapped.
I am not really sure we need "GStatus" or something like this. Maybe for the moment we just need a better name for GAccessor. I personally use G_ anyway so it does not matter.

If you want to dig into this deeper it would be awesome if you could look how the Python bindings of Gtk handeled all of this. From my perspective the user of Gtk.jl should not use the G_ module at all. All these functions need to be in the main module. The reason for GAccessor was (as far as I understood this) more a playground to look what we get from the automatic scripts.